This invention relates to a joining structure of a solid electrolytic element, especially of a solid electrolytic element used for an oxygen sensor.
Conventionally, a solid electrolytic detecting element is applied to an oxygen sensor for detecting the concentration of oxygen in gas. The solid electrolytic element is composed of a solid electrolytic panel which generates electromotive force due to its difference in oxygen concentration from that of the ambient atmosphere. A measuring electrode is provided on the surface of the solid electrolytic panel that faces the gas to be measured. A reference electrode is provided opposite to the measuring electrode. The measuring and the reference electrodes are connected, respectively, via electron conducting layers to output terminals on the end of the solid electrolytic panel. Around the electrodes there are heating elements which stabilize output by heating the solid electrolytic panel. The heating element is connected via electron conducting layers to heating terminals.
Lead wires, generally made of stainless or tin-plated copper, are connected respectively to the output terminals and the heating terminals so as to deliver output or to apply voltage.
Various joining structures of the solid electrolytic element have been proposed. The platinum lead wires are fixed to the terminals, for example, by using platinum paste or high-temperature solder, by sintering the platinum wires together with ceramic, or by plug engagement. However, these related-art joining methods cannot assure high reliability.
Another joining structure has been proposed in which the lead wires are directly soldered to the terminals on the surface of the solid electrolytic panel. This structure has high joining strength as well as high heat resistance. However, since the solid electrolytic panel has a small coefficient of thermal conductivity, it is hard to lower the gradient of the temperature of the terminals uniformly. When the temperature of the terminals lowers, the fluidity of soldering material is impaired, reducing the joining strength of the terminals and the lead wires. On the other hand, when the soldering temperature is raised to improve the fluidity of the soldering material, heat adversely affects the solid electrolytic panel because the reducing gas resulting in soldering disperses oxygen ions from solid electrolyte, which makes the solid electrolytic panel dark. When soldering material with a low melting temperature is applied to avoid the deterioration of the solid electrolyte due to high temperature, the heat resistance of the joining structure is deteriorated.